I'm scratching my head trying to figure this one out. Here's the story: I started brewing extracts, and they all came out great. I wanted to get into all-grain so a last year I ordered myself the supplies and have been trying to make a single good all-grain batch since then.
I am using the brew-in-a-bag method, and use pilsner malt, crystal malts, special b, flaked wheat, flaked oats, etc. Every beer, from the lightest pale ale (pils, crystal 40L, lots of hops) to darker beers (pils, special b) has come out with a weird flavor that I can only describe as "dark" and maybe burnt tasting, very bitter, not pleasant at all. Roasted malts used in stouts taste significantly better than this. The lightest beers, like a wit, taste dark and undrinkable. It kind of makes you pucker, but I can't really tell if it's astringent or sour or something else.
I've been doing one to three gallon batches. I strike at 165-170F or so, mash at 155F for an hour with a mash PH of around 5.0 according to my PH test strips. I'm using tap water, which I see others online say is okay (Seattle area). I've been mashing in my bottling bucket so that I can let a lot of the gunk settle out and use the spigot to pour into my pot for the boil, leaving behind most of the finer sediment that would otherwise be on the bottom of the pot with brew-in-a-bag. I boil for an hour with multiple hop additions based on the recipe, then cool in a bath of ice water, aerate by vigorously stirring and then pitch. The yeast always take off within a few hours and the fermentation goes great. Tasting the beer during bottling it tastes okay (I tend to try after the priming sugar is added, so it may mask the flavor), but by the time I drink the beer in a week or two it has this off flavor. I've tried letting the beer sit for a couple of months, but the flavor just stays and stays. I've tried both letting it sit in primary for a month and bottling after a week to let it sit in bottles for a month or two.
I'm really at a loss here and getting incredibly frustrated. Any suggestions would be really nice. I wish I knew what I was doing wrong. Feel free to try and help me put my finger on the exact terminology to use for this off flavor - I'm not all that experienced at describing them.
Edit: thought I would add that I don't have a grain mill. I have bought both pre-crushed and whole grains. Until I get a mill I've been using the blender, doing one cup at a time for 5-8 seconds on low to expose the kernels. Both the blender-chopped malt and the pre-crushed malt from a homebrew shop exhibit the off-flavor problem, so I can't imagine it's the crush.